Green roof sowing and aftercare
Seed of a suitable mixture (such as ER1 or ER1F) can be sown directly on to the prepared surface or sown in pots to be added as plants. Seed is best sown in the autumn unless irrigation is provided when seed can be sown in spring, summer or autumn.
Choice of species
When choosing species, raw soil forming materials like crushed concrete provide an ideal opportunity to select specialist plants which are adapted to periodic drought and nutrient stress but would not be able to compete when sown on typical topsoil.
Pioneer species known to be good at colonizing poor stony ground are good, as are species that can readily re-establish from seeds after drought. As initial establishment on poor materials can be slow, annual wild flowers can be added to also give flowers in the first season. Plants which are attractive or useful to insects add value to schemes. Tall species prone to wind damage are best avoided on exposed rooftops as are 'weedy' species with wind dispersed seeds that could cause problems by spreading to neighbouring properties. We have developed a seed mixture which is suitable for sowing on a wide range of substrates.
Grass or no grass?
Grasses are key components of many ecological systems and important food plants for a range of invertebrates. Many green roof designers however have a preference for mixtures based on wild flowers, with little or no grass component. These have been found to work well when sown onto poor, essentially sterile, substrates and avoid worries about competitive grasses taking over. However for many schemes it is still early days in terms of the progress of succession. Inevitably grasses will in time invade all sites. The decision to be taken at the design stage is whether to leave this to chance (accepting that 'weed' grasses are likely to be the first arrivals) or to pre-empt this by sowing a small quantity of short, attractive but non invasive grasses like Quaking grass and Crested hairgrass from the outset. The final choice of which grasses to include will be influenced by the nature, depth and history of the substrate. Poor, shallow, droughty materials may take a very long time to become grassy (if ever). Deeper topsoil based roofs in wetter regions will become grassy quite quickly so in these circumstances a turf roof approach is perhaps the more realistic or ecologically pragmatic solution and requires a suitable grass mixture with wild flowers and some aftercare (mowing).
To some extent green roofs have a historical precedent. In medieval and earlier times most roofs were covered with thatch or turf. Living turf roofs were the norm for low status dwellings and many thatched roofs were planted or turfed along their ridges. Modern construction methods have enabled a revival of this tradition without its failings. Grasses can be sown on roof schemes in most situations. On shallow 'soils' in regions with low rainfall, grasses will burn off in summer and cover will be sparse and open. In wetter regions or with irrigation a closed turf can develop which can be green all year round.
Aftercare of sown green and turf roofs
The requirement for aftercare will vary from scheme to scheme. Some will need minimal maintenance, perhaps only occasional tidying cuts for aesthetic reasons. Living roofs with reasonable growing conditions will produce growth that is likely to need annual maintenance for best results. This gardening will involve weeding and mowing along the lines of grassland aftercare. The practical and safety implications of maintenance work at height obviously need to be considered in advance of construction.